[foreign-memaccess+abi] RFR: 8278151: Heap segments should handle alignment constraints in a deterministic fashion [v2]

[Maurizio Cimadamore]

On Thu, 2 Dec 2021 14:55:48 GMT, Radoslaw Smogura <duke at openjdk.java.net> wrote:

>> src/java.base/share/classes/java/lang/invoke/X-VarHandleMemoryAccess.java.template line 119:
>> 
>>> 117:         long address = base + offset;
>>> 118:         long maxAlignMask = bb.maxAlignMask();
>>> 119:         if (skipAlignmentMaskCheck && maxAlignMask == 0) {
>> 
>> I've got minor comment. I wonder if we can skip `skipAlignmentMaskCheck` parameter and just base on `maxAlignMask`
>
> Sorry, it was stupid question. Please ignore

No problem - this is quite tricky. In general, `skipAlignmentMaskCheck` still does _something_ when accessing native segments (as the max align mask is zero there), but in general you are morally correct in observing that it's less useful than it used to be. My hope is that with the right VM fixes, we can get rid of `skipAlignmentMaskCheck` completely, and simplify all this.

For the records, the difference between unaligned and aligned access is as follows:


Benchmark                                                Mode  Cnt  Score   Error  Units
LoopOverNonConstant.segment_loop_instance                avgt   30  0.229 ? 0.002  ms/op
LoopOverNonConstant.segment_loop_instance_aligned        avgt   30  0.335 ? 0.001  ms/op


It's not terrible, but could of course be better.

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PR: https://git.openjdk.java.net/panama-foreign/pull/622